The present invention relates to the use of a pneumovirus NS1 protein and/or NS2 protein or a nucleic acid encoding pneumovirus NS1 protein and/or NS2 protein for the preparation of a pharmaceutical formulation for reducing the immune response mediated by interferon (IFN). The invention further relates to recombinant pneumoviruses, in particular respiratory syncytial viruses (RSV), having an increased, reduced, or lacking a resistance to the interferon (IFN) mediated immune response of the host, recombinant viruses having an increased resistance to the interferon (IFN) mediated immune response, and the use of these viruses in pharmaceutical formulations, e.g. as vaccines.